In OSPF, NSSA areas use Type-7 LSAs to describe external routes. When an NSSA ABR translates a Type-7 LSA to a Type-5 LSA (so it can be advertised into Area 0), it usually copies the forwarding address from the Type-7 LSA. With Forwarding Address Suppression enabled, the ABR sets the forwarding address in the Type-5 LSA to 0.0.0.0—regardless of the original Type-7 LSA’s forwarding address.

In large OSPF deployments, route summarization is used at area boundaries (ABRs) to reduce the size of routing tables in Area 0 (the backbone) and enhance network scalability. This is typically achieved using the area range command on ABRs. This command consolidates multiple more-specific prefixes from a non-backbone area, such as Area 2 and Area 8 in Figure, into a single summary route advertised into the backbone.

When an ASBR in an NSSA redistributes external routes, such as those from a connected network or another routing protocol (see ASBR in Area 2 in Figure), it generates Type-7 LSAs. These Type-7 LSAs are specific to NSSAs and are used to carry information about external routes within the NSSA. The NSSA ABR translates these Type-7 LSAs into Type-5 LSAs (Autonomous System External LSAs) before flooding them into the backbone (Area 0). See ABR connecting Area 2 to Area 0 in Figure. This allows the external routes to be propagated throughout the rest of the OSPF domain.

The Type-7 and Type-5 LSAs include a forwarding address—often the interface address of the ASBR that originated the external route. This address instructs OSPF devices where to forward traffic destined for the external prefix.

When the area range command is used, only the summary route is advertised into Area 0—not the specific prefixes or host routes within Area 2. If the forwarding address in a Type-5 LSA is a specific address within Area 2 that has been suppressed by summarization, backbone devices may not have a route to this address. As a result, the external prefix (though present in the routing table) becomes unreachable from the backbone, since traffic cannot be routed to the correct ASBR.

Before configuring this feature, consider the following caution.

Caution

Configuring this feature causes the device to be noncompliant with RFC 1587. In addition, suboptimal routing may occur if better paths exist to reach the destination’s forwarding address. Configure this feature only after careful consideration and once the network topology is understood.